Unless you live in Texas you can't. I was going to do the same but due to a Texas law you can't buy a car in Texas and take it out of state....unless you pay sales tax and license in Texas, then do it again where you live.
I wish Tesla would come out with something specifying how order changes will work. I was told by a Tesla office that when your order came up you would be notified and could make changes then without a new reservation number. How can the tri motor be first when there is a quad motor now too...